Initiator for surface-based polymerization and use thereof

ABSTRACT

Disclosed are polymerization initiators as may be utilized for addition of polymers to a substrate surface. The initiators are azo-based initiators that include multi-functionality through addition of multiple anchoring agents to an inner azo group. Disclosed polymerization initiators can be utilized to form high density and high molecular weight polymers on a surface such as a particulate surface. Formed materials can be beneficial in one embodiment in fracking applications, providing composite proppant/polymer materials that can prevent leakage of polymers from a subterranean geologic formation.

CROSS REFERENCE TO RELATED APPLICATION

This application claims filing benefit of U.S. Provisional PatentApplication Ser. No. 62/266,290 having a filing date of Dec. 11, 2015,which is incorporated herein by reference in its entirety.

BACKGROUND

In industrial applications involving polymeric materials, thesignificance of high yields, optimized processes and cost efficiency ofthe materials cannot be overstated. Typically, industrial polymers aresynthesized using condensation, free radical or anionic polymerizationmethods. When considering applications involving polymers bonded tosubstrate materials, there are several synthesis methods that are highlyeffective. The three main methods include physisorption, grafting-to andgrafting-from techniques. In physisorption (FIG. 1A), ligands range fromweak to strong adsorbents, and typically bind via an electrostatic orhydrogen bonding approach. In the grafting-to approach (FIG. 1B), thesubstrate is modified with a ligand bearing terminal functionality suchthat the functionality is available for post-modification. This methodis typically utilized for higher yields favouring smaller molecularweight polymers. In the grafting from approach (FIG. 1C), ligandscontaining chain transfer agents (CTAs) are commonly utilized followedby the growth of polymer chains from the surface. Highly dense brushescan be achieved by this method, as polymer growth is dependent upon thediffusion of monomers to the growing chain end.

One application of interest for industrial polymeric materials ishydraulic fracturing (also commonly referred to as hydrofracking orfracking), which has recently seen a surge in its application to newlyminted oil and gas fields. Although the technology has been known forseveral decades, recent improvements have made it economically feasibleto extract oil and gas “horizontally” via fracking, as opposed to themore common vertical drilling. Its adoption is more pronounced in theU.S., where more than 1.1 million active oil and gas wells span across36 states. There are, however, several concerns and difficulties in oilrecovery via fracking and in particular from shale deposits at depths ofseveral thousand feet.

Briefly, the process entails the creation of a hydraulic fracture in thegeologic formation through pumping of high viscosity fracturing fluidfor a short period (2-3 hours). The resulting high pressure exceeds therock formation strength and a fracture is created. The pathways thusformed allow the oil in the fractured formations to flow into thewellbore, which enables oil recovery at high rates. Fracturing fluidstypically contain a variety of additives that aid in fracture formation,delivery of proppants to the fracture zone and maintenance of goodconductivity such that the networks formed do not collapse/clog.Additives include viscosifiers (high molecular weight polymers),biocides, corrosion inhibitors, crosslinkers, friction reducers, gellingagents, scale inhibitors, surfactants and pH control agents. The exactrecipe for any fracturing fluid varies depending on the type and depthof the shale formation, borehole geometry, the amount of recoverablegas, etc. However, two main ingredients are a necessity: frictionreducers and proppant materials.

Friction exists between the fracturing fluid and the contact surface ofthe steel pipe and within the water itself (as turbulence) when thefluid is pumped. High pressure can overcome the contact friction, andfriction reducers are included to maintain non-turbulent flow. Frictionreducers typically include a high molecular weight polyacrylamidepolymer. In the presence of water, the polyacrylamide hydrates and itshydrodynamic radius increases, resulting in the prevention of turbulencein the moving water. Polyacrylamides are generally available as a drypowder, and are mixed with a mineral oil base fluid for stabilizationprior to addition to the fracturing fluid. The amount of frictionreducing materials typically ranges from 0.05-1% by weight of thefracturing fluid mixture.

Proppants are solid materials (generally treated sand or ceramicstructures) that aid in keeping the fracture open during the oilrecovery operation. The composition and geometry of the proppant canplay a large role in maintaining flow of the fluids through thefractures. For instance, untreated sand can cause significant fines tobe generated (due to crushing of the sand at high pressure) and may notmaintain the fracture as open. There has been a shift toward chemicallytreated sand as proppant, especially toward formation of treated sandthat can be both lightweight (to prevent settling) and high strength (toavoid being crushed).

One issue of concern in fracking is evacuation of the fracturing fluidfrom the established networks after oil recovery is completed. Once thepressure is released after pumping, about 60% of the fluid returns tothe wellbore and can be consequently recovered and reused. However,several thousand gallons of fracturing fluid can remain in the stratafollowing use. These retained fluids can slowly migrate to groundwatersources and/or to the surface, and may pose a significant environmentalproblem. For instance, polymers of the fracturing fluid (e.g.,polyacrylamide friction reducers) may not degrade easily and the monomerunits (acrylamide) are often classified as toxic contaminants ingroundwater. Hence, it is in the interest of the industry to havesolutions that directly address this issue and minimize risk in acost-effective manner. Typically, established methods involve chemicalor thermal decomposition of fracturing fluid remaining in the networks,followed by recovery into the well. However, the cleanup procedures varybetween drilling companies, and effective methods of monitoring thesesteps are uncertain.

Thus, a need exists for materials and methods that can retain polymericmaterials in desired locations during and following use. For instance, aneed exists for materials and methods that can prevent the migration ofpolymeric components of fracturing fluids out of established fracturenetworks after oil recovery is completed.

SUMMARY

Objects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

According to one embodiment, disclosed is a polymerization initiatorthat can be utilized for surface polymerization. More specifically, aninitiator can include an azo-group and first and second anchoring agentson different ends of the azo-group. In one embodiment, the first andsecond anchoring agents can be identical to one another, but this is nota requirement. In one embodiment, one or both of the anchoring agentscan include a thiazoline functionality or a succinimide functionality.

According to another embodiment, disclosed is a particle including thepolymerization initiator at a surface thereof. For instance, a particlefor use as a proppant, e.g., a treated silica particle or a ceramicparticle, can include the polymerization initiator at a surface thereof.

Also disclosed are methods for utilizing the polymerization initiator.For example, a polymer can be provided on the particle surface by use ofthe initiator. For instance, a polymer as may be utilized as a frictionreducer can be adhered according to a “grafting to” or a “grafting from”polymerization scheme at the initiator and the particle can be utilizedas a composite proppant/friction reducer in a fracturing fluid.

Other features and aspects of the present invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, which includesreference to the accompanying figures, in which:

FIG. 1A schematically illustrates a physisorption surface polymerizationscheme.

FIG. 1B schematically illustrates a grafting-to surface polymerizationscheme.

FIG. 1C schematically illustrates a grafting-from surface polymerizationscheme.

FIG. 2 illustrates a reaction scheme for bonding an initiator to aparticulate substrate surface.

FIG. 3 illustrates an embodiment including both ends of a difunctionalinitiator bonded to a substrate surface.

FIG. 4 graphically presents the optical transmittance of particulatedispersions comparing syloid particles grafted with various densities ofpolyacrylamide.

DETAILED DESCRIPTION

Reference now will be made to embodiments of the invention, one or moreexamples of which are set forth below. Each example is provided by wayof an explanation of the invention, not as a limitation of theinvention. In fact, it will be apparent to those skilled in the art thatvarious modifications and variations can be made in the inventionwithout departing from the scope or spirit of the invention. Forinstance, features illustrated or described as one embodiment can beused on another embodiment to yield still a further embodiment. Thus, itis intended that the present invention cover such modifications andvariations as come within the scope of the appended claims and theirequivalents. It is to be understood by one of ordinary skill in the artthat the present discussion is a description of exemplary embodimentsonly, and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied exemplary constructions.

The present disclosure is generally directed to polymerizationinitiators as may be utilized for addition of polymers to a substratesurface. More specifically, the initiators are activated azo initiatorsthat include multi-functionality through addition of multiple anchoringagents to an inner azo group. Disclosed initiators can be utilized topolymerize monomers and in one embodiment can provide high density andhigh molecular weight polymers on a surface such as a particulatesurface.

In one particular embodiment, the initiators and polymer/substratecomposites formed therefrom can be utilized in addressing concerns suchas chemical leakage of polymeric additives in fracturing fluids used infracking processes. However, it should be understood that thepolymerization initiators disclosed herein are in no way limited to suchapplications

By anchoring a polymeric additive of a fracturing fluid to a largesubstrate (e.g., a proppant particle), migration of the polymer in therock strata can be hindered. When considering silica based proppantmaterials, the polymerization initiator can be utilized in developmentof an optimized proppant in which chemically treated sand can be furthermodified with a polymeric additive. Beneficially, the polymerfunctionalized proppant can prevent migration of the polymer throughgeologic formations and soil to the surface and/or groundwater.

The controlled radical polymerization of poly(acrylic acid) on silicaparticles has been well documented, but comparative examples with otherpolymers as may be utilized in fracturing fluid such as poly(acrylamide)are limited. One of the main reasons includes difficulty in handling ofthe poly(acrylamide), as it is soluble only in water and in selectco-solvent mixtures. Beneficially, disclosed polymerization initiatorscan be designed for polymerization of multiple different polymers,including poly(acrylic acid) and poly(acrylamide) as well as otherpolymers that can be utilized as components in fracturing fluids. Assuch, the initiators can have broad applicability in fracking as well asother applications.

In one embodiment, the polymerization initiator can function as adifunctional azo-based free radical initiator. Free radicalpolymerization provided by use of the initiator can have severaladvantages, including tolerance to low O₂ concentrations, very fastpolymerization kinetics and inexpensive initiator compounds. Activationof the initiator can provide multiple anchoring agents (e.g., thiazolinegroups) on ends of the initiator, and substitution can be controlled soas to vary depending on the ratio of anchoring functionality on thesubstrate surface (e.g., amine) to the functionality of the azo-basedpolymerization initiator. For instance, when the ratio of surfaceanchoring functionality to initiator functionality is large, e.g.,greater than 1 or about 2 or greater, multiple anchoring ends of asingle initiator (or of multiple initiators) can be bonded to thesurface via the anchoring moiety of the surface. This can lead to higherdensity of surface polymer, as each azo group of an initiator canprovide two radicals upon decomposition of the azo group via, e.g.,heating, each of which can initiate polymerization. In cases where theratio of surface anchoring functionality to anchoring functionality ofthe initiator is less, e.g., about 1 or less, fewer anchoring ends ofthe initiator will bond to the surface, and thus fewer polymers willdevelop at the substrate surface. For example, if both ends of adifunctional initiator are anchored, the azo group initiator canencourage a polymer growth scheme carried out from the radical at eachseparately bonded end, developing two polymers from the substratesurface. In contrast, if only a single end of a difunctional initiatoris anchored, only a single polymer growth scheme will be carried outfrom the single radical of the anchored initiator.

Activated azo polymerization initiators can be utilized in oneembodiment to generate high molecular weight polymers (e.g., 800 kDa orgreater number average molecular weight in some embodiments) in a shortperiod of time. Such high molecular weight polymeric additives can be ofuse in a variety of applications and in one particular embodiment infracturing fluids, for instance as surface bonded viscosity modifierssuch as poly(acrylamide) that can be retained in the fracture network asa composite proppant/polymer construct.

The polymerization initiator can have the general structure as follows:X₁—R₁-A₁-N═N-A₂-R₂—X₂

A₁ and A₂ of the polymerization initiator can be the same as ordifferent from one another and are attachment groups that can includethe general formula:

in which R₃ and R₄ can be independently H, CN, CH₃, COOR₆ (R₆═H, alkyl,e.g., C1 to C10 alkyl), C₂H₅, etc.; and R₅ can be a bond or an alkylgroup, e.g., C1 to C10 alkyl.

R₁ and R₂ of the polymerization initiator can be the same as ordifferent from one another and can include any suitable linkage groupsuch as, without limitation, a carbonyl-containing linkage (e.g., ketonelinkage, ester linkage, amide linkage, acid anhydride linkage, imidelinkage, etc.), an amine-containing linkage (e.g., amide groups, azidegroups, cyanate groups; nitrate groups, nitrite groups, etc.), athiol-containing linkage (e.g., sulfinic acid, sulfonic acid,thiocyanates, etc.), a phosphonate linkage, an epoxy linkage,alkene-containing linkage, and so forth. Optionally, R₁ and/or R₂ can bea bond.

X₁ and X₂ of the polymerization initiator can be the same as ordifferent from one another and can include any anchoring agent as isgenerally known in the art. One or both of the anchoring agents can beattached to a surface for subsequent attachment of polymeric chains(e.g., via a “grafting-from” or “grafting-to” approach, as described ingreater detail below) via a radical of the polymerization initiator. Oneor both of the anchoring agents can be covalently bonded to a substratesurface, either directly or via a functionalization group. Theparticular anchoring agent(s) can be selected based upon the type ofsurface (e.g., a proppant nanoparticle) and/or the type of polymericchain(s) to be attached thereto.

In forming the polymerization initiator, the anchoring agent(s) can bereacted with an azo-containing compound. As such, the anchoring agent(s)can have a functional group for reaction, reaction of which can providethe linkage group of the polymerization initiator. Suitable functionalgroups can depend upon the particular azo compound and can include, butare not limited to, amine groups (e.g., amide groups, azide groups,cyanate groups; nitrate groups, nitrite groups, etc.), thiol groups(e.g., sulfinic acid, sulfonic acid, thiocyanates, etc.), phosphonategroups, hydroxyl groups (e.g., —OH), carboxylic acid groups (e.g.,—COOH), aldehyde groups (e.g., —CHO), halogen groups (e.g., haloalkanes,haloformyls, etc.), epoxy groups, alkenes, alkynes, and the like.

For example, in one embodiment, a mercaptothiazoline anchoring agent canbe reacted with an azo-containing compound such as4,4′-Azobis(4-cyanovaleric acid), a commercially available free radicalinitiator, at both of its carboxylic acid functional groups to form thepolymerization initiator per the following reaction scheme:

Other anchoring agents as may be reacted with an azo-containing group toform the polymerization initiator can include anchoring agents as areknown in the art. For instance, in one embodiment, 4-cyanopentanoic aciddithiobenzoate (CPDB) can be reacted at one or both ends of adifunctional azo-containing compound to form the polymerizationinitiator. The CPDB end(s) of the polymerization initiator can then beattached to an amine-functionalized substrate surface by activating theends using, e.g., 2-mercaptothiazoline. The polymerization initiator canthen be immobilized onto the surface via a condensation reaction withthe amine groups on the substrate surface.

In one embodiment, the polymerization initiator can have the followingstructure:

It should be understood that the anchoring agent for use in forming thepolymerization initiator is not limited to thiazoline-containing agents,and other compounds are encompassed herein. For example, the anchoringagent component can include succinimide reactivity in one embodiment.For instance, N-hydroxysuccinimide (NHS) can be reacted with anazo-containing compound such as 4,4′-Azobis(4-cyanovaleric acid) to forma polymerization initiator.

Following formation, the polymerization initiator can be immobilizedonto a surface (e.g., colloidal silica nanoparticles), for instance viathe thiazoline groups of the above scheme. For instance, the dualfunctionalized polymerization initiator can be attached on the surfaceby first functionalizing the surface with amine groups using, e.g.,3-aminopropyldimethylethoxysilane. Use of a mono-functional silane suchas 3-aminopropyldimethylethoxysilane is not required, but it may bebeneficial in some embodiments as use of such as compared to atrifunctional silane can ensure the formation of a monolayer ofinitiator on the surface and can prevent particle agglomeration bycrosslinking during processing.

As mentioned previously, the ratio of the surface anchoring sites topolymerization initiator can be useful in determining the graftingdensity. Thus, the concentration of surface anchoring sites can beadjusted as desired, e.g., by varying the concentration of amino-silane.Addition of a small amount of an inert dimethylmethoxy-n-octylsilane canoptionally be utilized to partially cover the surface by inert alkylgroups and can help to tune the grafting density along with preventingaggregation of substrate particles.

The attachment scheme utilized to bond the polymerization initiator ontoan amine-functionalized surface can depend upon the anchoring agent usedin forming the initiator. For instance, when considering a thiazolinecompound such as the mercaptothiazoline described above, the initiatorcan be immobilized on the surface via a condensation reaction with aminegroups present on the surface as illustrated in the reaction scheme ofFIG. 2. Of course, the polymerization initiator can be activated asnecessary for the immobilization reaction according to any suitablereaction chemistry.

As previously stated, in those embodiments in which the surface includesa high concentration of anchoring sites relative to the amount of thepolymerization initiator, the initiator can react with the anchoringsites at both ends of the difunctional initiator. For instance, whenconsidering the thiazoline-functionalized azo-containing polymerizationinitiator illustrated above, the polymerization initiator can bind asurface at two sites as illustrated in FIG. 3.

In this embodiment, the single initiator can be utilized to provide twonitrogen radicals and as such two polymers at the surface, which canprovide a surface with a high polymer density.

The polymerization initiator can be utilized on a variety of differenttypes of surfaces. In one particular embodiment, the polymerizationinitiator can be utilized in conjunction with particles. The particlemay comprise, for example, natural or synthetic clays (including thosemade from amorphous or structured clays), inorganic metal oxides (e.g.,silica, alumina, and the like), latexes, organic particles, etc.Particularly suitable particles include inorganic particles, such assilica, alumina, titania (TiO₂), indium tin oxide (ITO), CdSe, etc., ormixtures thereof. Suitable organic particles include polymer particles,carbon, graphite, graphene, carbon nanotubes, virus nanoparticles, etc.,or mixtures thereof. The particles can be micro-scale particles ornano-scale particles.

In one particular embodiment, the polymerization initiator can beapplied to a surface of a proppant particulate substrate as may beutilized in a fracking process. Proppant particulate substrates caninclude, without limitation, graded sand, resin coated sand, bauxite,ceramic materials, glass materials, walnut hulls, polymeric materials,resinous materials, rubber materials, and the like, and combinationsthereof. A particulate surface can include specialty proppants, such asceramics, bauxite, and resin coated sand.

Combinations of different types of particles are also encompassed. Forinstance, by combining sand with a specialty proppant, a proppantinjection can have desirable strength, permeability, suspension, andtransport properties.

In some embodiments, the substrates can include naturally occurringmaterials, for example nutshells that have been chipped, ground,pulverized or crushed to a suitable size (e.g., walnut, pecan, coconut,almond, ivory nut, brazil nut, and the like), or for example seed shellsor fruit pits that have been chipped, ground, pulverized or crushed to asuitable size (e.g., plum, olive, peach, cherry, apricot, etc.), or forexample chipped, ground, pulverized or crushed materials from otherplants such as corn cobs. In some embodiments, the substrates can bederived from wood or processed wood, including but not limited to woodssuch as oak, hickory, walnut, mahogany, poplar, and the like. In someembodiments, aggregates can be formed, using an inorganic materialjoined or bonded to an organic material. In general, the proppantparticulate substrates can be comprised of particles (whether individualsubstances or aggregates of two or more substances) having a size in theorder of mesh size 4 to 100 (US Standard Sieve numbers). As used herein,the term “particulate” includes all known shapes of materials withoutlimitation, such as spherical materials, elongate materials, polygonalmaterials, fibrous materials, irregular materials, and any mixturethereof.

In some embodiments, the particulate substrate can be formed as acomposite from a binder and a filler material. Suitable filler materialscan include inorganic materials such as solid glass, glass microspheres,fly ash, silica, alumina, fumed carbon, carbon black, graphite, mica,boron, zirconia, talc, kaolin, titanium dioxide, calcium silicate, andthe like. In certain embodiments, a proppant particulate substrate canbe reinforced to increase resistance to the high pressure of theformation which could otherwise crush or deform the particles.Reinforcing materials can be selected from those materials that are ableto add structural strength to the proppant particulate substrate, forexample high strength particles such as ceramic, metal, glass, sand, andthe like, or any other materials capable of being combined with anotherparticulate substrate to provide it with additional strength.

In certain embodiments, the proppant particulate substrate can befabricated as an aggregate of two or more different materials providingdifferent properties. For example, a core particulate substrate havinghigh compression strength can be combined with a buoyant material havinga lower density than the high compression-strength material. Thecombination of these two materials as an aggregate can provide a coreparticle having an appropriate amount of strength, while overall havinga relatively lower density. As a lower density particle, a particle canbe suspended in a less viscous fracturing fluid, allowing the fracturingfluid to be pumped more easily, and allowing more dispersion of theproppants within the formation as they are propelled by the less viscousfluid into more distal regions. High density materials used as proppantparticulate substrates, such as sand, ceramics, bauxite, and the like,can be combined with lower density materials such as hollow glassparticles, other hollow core particles, certain polymeric materials, andnaturally-occurring materials (nut shells, seed shells, fruit pits,woods, or other naturally occurring materials that have been chipped,ground, pulverized or crushed), yielding a less dense aggregate thatstill possesses adequate compression strength.

Aggregates suitable for use as proppant particulate substrates can beformed using techniques to attach the two components to each other. Asone preparation method, a proppant particulate substrate can be mixedwith the buoyant material having a particle size similar to the size ofthe proppant particulate substrates. The two types of particles can thenbe mixed together and bound by an adhesive, such as a wax, aphenol-formaldehyde novolac resin, etc., so that a population of doubletaggregate particles are formed, one subpopulation having a proppantparticulate substrate attached to another similar particle, onesubpopulation having a proppant particulate substrate attached to abuoyant particle, and one subpopulation having a buoyant particleattached to another buoyant particle. The three subpopulations could beseparated by their difference in density: the first subpopulation wouldsink in water, the second subpopulation would remain suspended in theliquid, and the third subpopulation would float.

In other embodiments, a proppant particulate substrate can be engineeredso that it is less dense by covering the surface of the particulatesubstrate with a foamy material. The thickness of the foamy material canbe designed to yield a composite that is effectively neutrally buoyant.To produce such a coated proppant particulate, a particle having adesirable compression strength can be coated with one reactant for afoaming reaction, followed by exposure to the other reactant. With thetriggering of foam formation, a foam-coated proppant particulate will beproduced. The polymerization initiator can then be adhered to thecoating of the particulate.

As an example, a water-blown polyurethane foam can be used to provide acoating around the particles that would lower the overall particledensity. To make such a coated particle, the particle can be initiallycoated with Reactant A, for example a mixture of one or more polyolswith a suitable catalyst (e.g., an amine). This particle can then beexposed to Reactant B containing a diisocyanate. The final foam willform on the particle, for example when it is treated with steam whilebeing shaken; the agitation will prevent the particles fromagglomerating as the foam forms on their surfaces.

In one embodiment, a particulate substrate can have an average particlesize of about 1 to about 1000 nanometers, or 2 to about 750 nanometersin some embodiments. That is, the nanoparticles have a dimension (e.g.,a diameter or length) of about 1 to 1000 nm. Nanotubes can includestructures up to 1 centimeter long, alternatively with a particle sizefrom about 2 to about 50 nanometers.

A particulate substrate may be crystalline or amorphous. A single typeof particle may be used, or mixtures of different types of particles maybe used. Non-limiting examples of suitable particle size distributionsof nanoparticles are those within the range of about 2 nm to less thanabout 750 nm, alternatively from about 2 nm to less than about 200 nm,and alternatively from about 2 nm to less than about 150 nm.

It should also be understood that certain particle size distributionsmay be useful to provide certain benefits, and other ranges of particlesize distributions may be useful to provide other benefits (forinstance, color enhancement requires a different particle size rangethan the other properties). The average particle size of a batch ofparticles may differ from the particle size distribution of thoseparticles. For example, a layered synthetic silicate can have an averageparticle size of about 25 nanometers while its particle sizedistribution can generally vary between about 10 nm to about 40 nm.

In one embodiment, a particulate substrate can be exfoliated from astarting material to form the particles. Such starting material may havean average size of up to about 50 microns (50,000 nanometers). Inanother embodiment, the nanoparticles can be grown to the desiredaverage particle size.

In general either of two methods can be utilized to form polymericchains extending from the composite including the substrate and thepolymerization initiator: a “grafting-from” approach of a “grafting-to”approach.

“Grafting-From” Methods

In one embodiment, the polymeric chain can be formed by polymerizing afirst plurality of first monomers on the polymerization initiator,resulting in the first polymeric chain being covalently bonded to thesubstrate via the initiator including the anchoring compound. Accordingto this method, the polymerization of the polymeric chain can beconducted through any suitable type of free radical polymerization, suchas reversible addition-fragmentation chain transfer (RAFT)polymerization, atom transfer radical polymerization (ATRP), etc.

The particular types of monomer(s) and/or polymerization technique canbe selected based upon the desired polymeric chain to be formed. Forexample, for RAFT polymerization, monomers containing acrylate,methacrylate groups, acrylamides, styrenics, etc., are particularlysuitable for formation of the polymeric chain.

Thus, the “grafting-from” method involves formation of the polymericchain onto the substrate surface via the anchoring compound of thepolymerization initiator and results in the polymeric chain beingcovalently bonded to the nanoparticle via the anchoring compound.

“Grafting-To” Methods

In one embodiment, the polymeric chain can be first polymerized andsubsequently covalently bonded to the surface of the nanoparticle viathe polymerization initiator. In this embodiment, the polymeric chain isnot limited to the types of monomer(s) capable of being polymerizeddirectly to the polymerization initiator. As such, as long as thepolymeric chain defines a functional group that can react and bond tothe polymerization initiator, e.g., via the nitrogen radical of thepolymerization initiator, any polymeric chain can be bonded to thenanoparticle.

In one embodiment, upon attachment, the polymeric chain can bedeactivated to prevent further polymerization thereon. For example, ifthe “grafting-from” method is utilized to attach the polymeric chain tothe surface via polymerization through a controlled livingpolymerization (CLP) technique (e.g., RAFT), a deactivation agent can beutilized, e.g., attached to the end of each polymeric chain, to inhibitfurther polymerization thereon. The deactivation agents can be selectedbased upon the type of polymerization and/or the type(s) of monomersutilized, but can generally include but are not limited to amines,peroxides, or mixtures thereof.

On the other hand, if the “grafting-to” method is utilized to attach thepolymeric chain to the surface via attaching a pre-formed firstpolymeric chain, the polymeric chain can be deactivated either prior toor after covalently bonding the polymeric chain to the polymerizationinitiator. For instance, an active polymerization end of the polymericchain can be deactivated prior to covalently bonding a second end of thepolymeric chain to the surface.

The deactivation of the polymeric chain can be achieved by any suitableprocess. In one embodiment, the polymer chain can be cleaved.Alternatively, the end of the polymer chain can be functionallydeactivated. For example, when formed via RAFT polymerization, the typesof reactions that can be used to convert RAFT agents to deactivated endgroups include reactions with diazo compounds, reactions withnucleophilic reagents such as primary amines, and reactions withoxidation agents which cleave the RAFT agent off the chain end and forman oxidized sulfur group such as sulfonic acid.

As mentioned, the polymeric chain(s) can be formed via a grafting toapproach in a controlled polymerizations, such as controlled livingpolymerizations (CLPs) or controlled ring-opening polymerizations, whichmay be independently selected based upon the particular anchoringagent(s)/initiators present on the surface, type of monomer(s) used toform the polymeric chain, and/or desired properties of the polymericchains formed. Through the use of these controlled polymerizations, eachpolymeric chain can be produced with desired polydispersity andarchitecture.

Controlled living polymerization generally refers to chain growthpolymerization which proceeds with significantly suppressed terminationor chain transfer steps. Thus, polymerization in CLP proceeds until allmonomer units have been consumed or until the reaction is terminated(e.g., through quenching and/or deactivating), and the addition ofmonomer results in continued polymerization, making CLP ideal for blockpolymer and graft polymer synthesis. The molecular weight of theresulting polymer is generally a linear function of conversion so thatthe polymeric chains are initiated and grow substantially uniformly.Thus, CLPs provide precise control on molecular structures,functionality and compositions. Thus, these polymers can be tuned withdesirable compositions and architectures.

Controlled living polymerizations and controlled ring-openingpolymerizations are generally known to those skilled in the art. A briefgeneral description of each technique is below, and is provided forfurther understanding of the present invention, and is not intended tobe limiting:

RAFT Polymerization

As previously mentioned, RAFT polymerization (RAFT) is one type ofcontrolled radical polymerization as may be carried out by use of thepolymerization initiators. RAFT polymerization uses thiocarbonylthiocompounds, such as dithioesters, dithiocarbamates, trithiocarbonates,and xanthates, in order to mediate the polymerization via a reversiblechain-transfer process. RAFT agents are generally thiocarbonylthiocompounds, such as generally shown below:

in which the z group primarily stabilizes radical species added to theC═S bond and the R group is a good homolytic leaving group. For example,the z group can be an aryl group (e.g., phenyl group, benzyl group,etc.), an alkyl group, etc. The R″ group can be an organic chainterminating with a carboxylic acid group.

RAFT polymerization can be performed by simply adding a chosen quantityof appropriate RAFT agents to the free radical polymerization. RAFTpolymerization is particularly useful with monomers having a vinylfunctional group (e.g., a (meth)acrylate group).

Typically, a RAFT polymerization system includes the monomer, theinitiator, and a RAFT agent (also referred to as a chain transferagent). Because of the low concentration of the RAFT agent in thesystem, the concentration of the initiator can be lower than inconventional radical polymerization.

RAFT is a type of living polymerization involving a conventional radicalpolymerization in the presence of the chain transfer reagent. Like otherliving radical polymerizations, there is minimized termination step inthe RAFT process. The reaction is started by the radical initiator(e.g., a nitrogen radical of the disclosed initiators). In thisinitiation step, the initiator reacts with a monomer unit to create aradical species which starts an active polymerizing chain. Then, theactive chain reacts with the thiocarbonylthio compound of the RAFTagent, which kicks out the homolytic leaving group (R″). This is areversible step, with an intermediate species capable of losing eitherthe leaving group (R″) or the active species. The leaving group radicalthen reacts with another monomer species, starting another activepolymer chain. This active chain is then able to go through theaddition-fragmentation or equilibration steps. The equilibration keepsthe majority of the active propagating species into the dormantthiocarbonyl compound, limiting the possibility of chain termination.Thus, active polymer chains are in equilibrium between the active anddormant species. While one polymer chain is in the dormant stage (boundto the thiocarbonyl compound), the other is active in polymerization.

By controlling the concentration of initiator and thiocarbonylthiocompound and/or the ratio of monomer to thiocarbonylthio compound, themolecular weight of the polymeric chains can be controlled with lowpolydispersities.

Depending on the target molecular weight of final polymers, the monomerto RAFT agent ratios can range from about less than about 10 to morethan about 1000 (e.g., about 10 to about 1,000). Other reactionparameters can be varied to control the molecular weight of the finalpolymers, such as solvent selection, reaction temperature, and reactiontime. For instance, solvents can include conventional organic solventssuch as tetrahydrofuran, toluene, dimethylformamide, anisole,acetonitrile, dichloromethane, etc. The reaction temperature can rangefrom room temperature (e.g., about 20° C.) to about 120° C. The reactiontime can be from less than about 1 h to about 48 h.

The RAFT process allows the synthesis of polymers with specificmacromolecular architectures such as block, gradient, statistical,comb/brush, star, hyperbranched, and network copolymers (see, e.g., U.S.Pat. No. 8,865,796 and U.S. Patent Application Publication Nos.2015/0266990, 2015,0073109, and 2014/0090850, all of which areincorporated herein by reference thereto.)

Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is another example of aliving radical polymerization as may be carried out by use of thedisclosed polymerization inititators. The control is achieved through anactivation-deactivation process, in which most of the reaction speciesare in dormant format, thus significantly reducing chain terminationreaction. The four major components of ATRP include the monomer,initiator, ligand, and catalyst. ATRP is particularly useful monomershaving a vinyl functional group (e.g., a (meth)acrylate group).

The catalyst can determine the equilibrium constant between the activeand dormant species during polymerization, leading to control of thepolymerization rate and the equilibrium constant. In one particularembodiment, the catalyst is a metal having two accessible oxidationstates that are separated by one electron, and a reasonable affinity forhalogens. One particularly suitable metal catalyst for ATRP is copper(I).

The ligands can be, e.g., linear amines or pyridine-based amines.

Depending on the target molecular weight of final polymers, the monomerto initiator ratios can range from less than about 10 to more than about1,000 (e.g., about 10 to about 1,000). Other reaction parameters can bevaried to control the molecular weight of the final polymers, such assolvent selection, reaction temperature, and reaction time. Forinstance, solvents can include conventional organic solvents such astetrahydrofuran, toluene, dimethylformamide, anisole, acetonitrile,dichloromethane, etc. The reaction temperature can range from roomtemperature (e.g., about 20° C.) to about 120° C. The reaction time canbe from less than about 1 h to about 48 h.

Ring-Opening Metathesis Polymerization

Ring-opening metathesis polymerization (ROMP) is a type of olefinmetathesis polymerization. The driving force of the reaction is reliefof ring strain in cyclic olefins (e.g. norbornene or cyclopentene) inthe presence of a catalyst. The catalysts used in a ROMP reaction caninclude a wide variety of metals and range from a simple RuCl₃/alcoholmixture to Grubbs' catalyst.

The monomer can include a strained ring functional group, such as anorbornene functional group, a cyclopentene functional group, etc. toform the polymeric chains. For example, norbornene is a bridged cyclichydrocarbon that has a cyclohexene ring bridged with a methylene groupin the para position.

The ROMP catalytic cycle generally requires a strained cyclic structurebecause the driving force of the reaction is relief of ring strain.After formation of the metal-carbene species, the carbene attacks thedouble bond in the ring structure forming a highly strainedmetallacyclobutane intermediate. The ring then opens giving thebeginning of the polymer: a linear chain double bonded to the metal witha terminal double bond as well. The new carbene reacts with the doublebond on the next monomer, thus propagating the reaction.

Ring-Opening Polymerization

In one particular embodiment, where the monomer includes a strained ringfunction group (e.g., a caprolactone or lactide), ring-openingpolymerization (ROP) may be used to form the polymeric chain. Forexample, a caprolcatone-substituted monomer is a polymerizable ester,which can undergo polymerization with the aid of the initiator and atin-based reagent as a catalyst.

EXAMPLES

Mercatotiazoline-activated ACVA was formed. 4,4′-Azobis(4-cyanovalericacid) (20 g, 35.679 mmol) was dissolved in 400 ml of 1,4-dioxane in a1000 ml round bottom flask. 2-Mercaptothiazoline (22.58 g, 188.38 mmol)and 4-dimethyl aminopyridine (0.5 g, 4.07 mmol) were added and themixture was stirred until complete dissolution of the mixture wasobserved. The mixture was cooled to 0° C., and in a separate 500 mlbeaker, N,N′-dicyclohexylcarbodiimide (34 g, 164.84 mmol) was stirred in100 ml of 1,4-dioxane until a cloudy suspension was observed. Thecarbodiimide solution was added dropwise to the mixture in the roundbottom flask at 0° C. via an addition funnel. The mixture was stirredovernight, and then filtered, followed by removal of the solvent throughrotary evaporation. The yellow solid was then purified throughrecrystallization in a 70% hexane/ethyl acetate mixture (31.5 g, 91.6%yield). ¹H-NMR (300 MHz, CDCl₃): δ (ppm) 4.59 (t, 2H), 3.30 (t, 2H),3.12-3.63 (m, 2H), 2.41-2.64 (m, 2H), 1.75 (s, 3H).

NHS-activated ACVA was formed. 4,4′-Azobis(4-cyanovaleric acid) (5 g,17.84 mmol) was dissolved in 100 ml of THF in a 500 ml round bottomflask. N-hydroxysuccinimide (4.93 g, 42.81 mmol) and 4-dimethylaminopyridine (0.4 g, 3.56 mmol) were added and the mixture was stirreduntil complete dissolution of the mixture was observed. The mixture wascooled to 0° C., and in a separate 500 ml beaker,N,N′-dicyclohexylcarbodiimide (8.8 g, 42.81 mmol) was stirred in 50 mlof THF until a cloudy suspension was observed. The carbodiimide solutionwas added dropwise to the mixture in the round bottom flask at 0° C. viaan addition funnel. The mixture was stirred over night, and thenfiltered, followed by removal of the solvent through rotary evaporation.The yellow solid was then purified through recrystallization in a 70%hexane/ethyl acetate mixture (7.60 g, 90% yield). ¹H-NMR (300 MHz,CDCl₃): δ (ppm), 3.12-3.63 (m, 2H), 2.7 (t, 4H), 2.41-2.64 (m, 2H), 1.75(s, 3H).

Amino functionalized syloid particles were formed. A THF solution (50mL) of bare syloid particles (0.5 g) was mixed in a 200 ml round bottomflask and stirred for 10 min. 3-Aminopropylsilane (50 μl, 0.265 mmol)was added to the mixture and the flask was equipped to a condenser andpurged with N₂. The mixture was refluxed overnight under N₂. The mixturewas diluted with 100 ml of THF and then centrifuged at 3000 rpm for 5min. The particles were isolated and suspended in 50 ml of THF. Thecentrifugation-redispersion process was repeated two more times, and inthe final step, the isolated particles were stored as a dichloromethane(DCM) solution (20 ml).

Activated ACVA (0.128 g, 0.266 mmol) was dissolved in THF (7 ml) addedto the amino functionalized syloid in DCM (20 ml) at 0° C. Aftercomplete addition, the solution was stirred over night at roomtemperature under nitrogen. The reaction mixture was then diluted into alarge amount of THF (100 mL). The particles were recovered bycentrifugation at 3000 rpm for 15 min. The particles were thenresuspended in 100 mL of THF. This centrifugation-dilution procedure wasrepeated another two times until the supernatant layer aftercentrifugation was colorless. The yellow azo functionalized syloidparticles were then dried in the oven for 1 h, and then stored in thefreezer in an air-tight container.

In a dried Schlenk tube, azo anchored syloid (0.05 g) was dissolved inTHF (2 mL). Methyl methacrylate (MMA) (2 mL) was then added to the tube.The mixture was degassed by three freeze-pump-thaw cycles, back filledwith nitrogen, and then placed in an oil bath preset at 65° C. Thepolymerization was quenched by submersion of the reaction vessel in icewater. The polymer solution was precipitated into hexanes, andredispersed in THF. The precipitation-redispersion process was repeatedonce more to obtain poly(methyl methacrylate) functionalized syloid.

For the polymerization of styrene, the above reaction was used with thesame ratio of reactants, but MMA was substituted with styrene (2 mL).The polymer solution was precipitated into methanol, and redispersed inTHF. The precipitation-redispersion process was repeated once more toobtain poly(styrene) (PSt) functionalized syloid. The polymerfunctionalized syloid was then subjected to cleavage of the polymersaccording to the procedure in 5.2.10. GPC analysis of the cleaved PMMAprovided Mn: 342 k, PDI: 2.12 and that of the cleaved PSt with Mn: 681k, PDI: 2.81.

Filter columns were made with a plug of cotton, sand (1 mm) and filtermaterial (4 cm) in a Fisherbrand™ Disposable Borosilicate Glass PasteurPipette (length: 5.75 in., 146 mm). Filter materials used include Syloidparticles, silica gel (70-200 um), and diatomaceous earth. A typicalmethod involved the dissolution of the sample (14-78 mg) in water (1 ml)and its addition to the column to be filtered into a vial. The vial wasthen freeze-dried to calculate the mass of its contents. All experimentsinvolved three sets of samples including the control (1 ml of water),free polymer (for PAA, Mn: 1800 g/mol) and Syloid-polymer. Retentionefficiencies were calculated based on the amount of Syloid-polymer andfree polymer had passed through the column compared to the originalamounts of each used. A retention efficiency of 0% for the free polymerindicated that all of the free polymer had passed through the column.Conversely, a retention efficiency of 100% for the Syloid-polymerindicated that none of the Syloid-polymer had passed through the column.All filtration tests were accompanied with a control test, where water(1 mL) was passed through the same mass of the filter material in aseparate piptette. As some of the filter material managed to passthrough the cotton plug, the mass of the filter material in the vial(after freeze-drying) was calculated and compared with the run withsyloid-polymer/polymer. This control test was used to recalculate anadjusted retention efficiency, which accounts for any filter materialthat passes through the cotton plug.

In order to simulate real world conditions, extended washing procedureswere tested to see the retention efficiency after several additions ofwater to the column. In this procedure, samples were dissolved in water(2 mL), passed through the column, and followed up with two separateadditions of water (1 mL) each. Similar to the procedure outlined above,each experiment included a control (2 mL of water), and the free polymerand syloid-polymer respectively.

In order to further test the proof of concept for the free radicalpolymerization, several polymerizations were performed with styrene andmethyl methacrylate without the presence of any free initiator (Table 1,below). In all cases where the ratio of monomer to solvent was 1:1 (mL),the polymerization proceeded very quickly and gelled composites wereobtained. The composites were dissolved in THF and then subject to HF tocleave the polymers for analysis by GPC. GPC traces revealed high MWpolymers and high polydispersities (2.12-2.81), both relating to theuncontrolled characteristics of the free radical polymerization.

TABLE 1 Syloid-azo (g) Monomer Time (h) Mn (GPC) PDI 0.05 MMA (2 mL)2.22 342k 2.12 0.05 Styrene (2 mL) 2.1 681k 2.81

Surface initiated free radical polymerizations of water solublepolyacrylamide were performed in aqueous conditions. In a 1,000 ml3-neck round bottom flask, ACVA functionalized syloid particles (0.5 g)were suspended in 600 ml DI water. The suspension was sparged with alarge flow of N₂ for 30 minutes before adding acrylamide in varyingamounts. The solution was heated to 80° C. and stirred for 24 hours. Thesolution was allowed to cool to room temperature before recovering thePAM grafted particles by centrifuging at 3,000 rpm for five minutes. Theparticles were then suspended back into water and recovered two moretimes. Content of surface polymer was measured using thermogravimetricanalysis (TGA). Table 2, below shows the percent weight loss of samplespolymerized with varying monomer to grafted initiator ratios.

TABLE 2 Equivalents of % wt loss at Sample acrylamide to initiator 800°C. Bare Syloid NA 3 Syloid-ACVA NA 13 MHB2-198  204 12 MHB2-202 1019 28MHB2-205 1631 35

To test the efficacy of suspending polymer grafted particles in aqueousconditions, experiments were performed to measure the relative amount ofparticles in solution over time. Syloid (100 mg) was diluted into 20 mlwater yielding a 5 mg/ml Syloid in water solution for each sample.Syloid loading of PAM grafted particles was determined through TGAanalysis. For preparation of free PAM+bare Syloid solution, a portion offree polymer recovered from MHB2-205 was dried before combining the freepolymer (25 wt %) and bare Syloid (100 mg) in 20 ml water to make theresulting Syloid+bare PAM solution. The samples were sonicated for 20minutes before transferring an aliquot of each sample to a polystyrenecuvette. A lid was placed on the cuvette and sealed in place withparafilm. Transmittance at 300 nm was measured immediately (t=0) thenagain at timed intervals for 100 hours. A plot of time vs. transmittanceis shown in FIG. 4.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood the aspects of the various embodiments may beinterchanged both in whole and in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in the appended claims.

What is claimed:
 1. A polymerization initiator comprising the structureof:X₁—R₁-A₁-N═N-A₂-R₂—X₂ in which A₁ and A₂ are the same as or differentfrom one another and each comprise an attachment group, R₁ and R₂ arethe same as or different from one another and each comprise a linkagegroup, X₁ and X₂ are first and second anchoring agents, respectively,wherein X₁ and X₂ are the same as or different from one another and eachindependently comprises a thiol group, a phosphonate group, or ahalogen.
 2. The polymerization initiator of claim 1, wherein A₁ and A₂have the general structure:

in which R₃ and R₄ independently comprise H, CN, CH₃, COOR₆ (R₆═H oralkyl), or C₂H₆; and R₅ is an alkyl group.
 3. The polymerizationinitiator of claim 1, wherein R₁ and R₂ independently comprise acarbonyl-containing linkage, an amine-containing linkage, athiol-containing linkage, a phosphonate linkage, an epoxy linkage, analkene-containing linkage, or are a bond.
 4. The polymerizationinitiator of claim 1, wherein the first anchoring agent and/or thesecond anchoring agent comprises a thizoline group.
 5. Thepolymerization initiator of claim 1, wherein the first anchoring agentand/or the second anchoring agent comprises a succinimide group.
 6. Asubstrate comprising a surface and the polymerization initiator of claim1 bonded to the surface.
 7. A polymerization initiator having thefollowing structure:


8. A substrate comprising the polymerization initiator of claim
 7. 9. Apolymerization initiator having the following structure:


10. A substrate comprising the polymerization initiator of claim 9.